What is Father’s motivation for keeping the truth about Mother away from Christopher? Is his purpose to save Christopher’s feelings or to punish his wife?

I think Father's motivations are a bit of both. He's been left to raise a child, who is more than a little challenging.... his wife walked out. His refusal to allow her back into Christopher's life is very much a punishment, but it's also likely he doesn't trust her to walk out on him again.